[cfe-dev] Applying custom filters after preprocessing (and compile-time sprintf optimization)

[Richard Smith]

On Thu, Oct 2, 2014 at 5:18 PM, Kazuho Oku <kazuhooku at gmail.com> wrote:

> Hi,
>
> Recently I have started writing a preprocessor that wraps the C
> preprocessor for optimizing calls to sprintf, since it is the one of the
> bottlenecks of my application.
> https://github.com/kazuho/qrintf
>
> For GCC, it was easy for me to override the default C preprocessor, by
> using its `-no-integrated-cpp` and `-wrapper` options (how I have done it
> can be found in the `qrinf-gcc` and `qrintf-gcc-wrapper` files included in
> the above repository).
>
> However I have so far been unable to find a similar command-line option
> for Clang.  Is there any way to post-process the output of the preprocessor
> in Clang?  Or do I need to modify the source code of Clang itself?
>
> Thank you in advance.
>
> PS. BTW it would be great if compile-time optimization of sprintf goes
> into Clang.  Is there any chance of such possibility?
>

In the general case, it's not possible to optimize sprintf fully, because
its behavior depends on the current locale (this affects %d and %u, which
it looks like you optimize) and it might have libc-specific extensions, but
LLVM does optimize in some simple cases: for instance, a sprintf containing
no % is optimized into memcpy. I expect patches would be accepted to
implement further cases, such as handling %s and %c.
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